The pipettes currently available on the market, often called micropipettes, are syringes including a cylinder extended by a shaft and a dispensing tip and a piston sliding in the cylinder. This piston can be actuated, manually or by a motor, over a travel determined by the operator using a suitable counter.
During operation, the piston is pushed down to its end position, the dispensing tip mounted at the end of the shaft is dipped into the liquid to be sampled, then the piston is moved up by a distance corresponding to the volume of liquid displayed on the counter. The upward movement of the piston causes the desired volume of liquid to be aspirated only into the tip, the liquid being then able, via another downward movement of the piston, to be discharged into a target reserved therefore.
Pipettes of this type, both manual and motorized, are disclosed, for example, in U.S. Pat. Nos. 5,983,733, 6,170,343 and 6,254,832. Multishaft pipettes are also disclosed, for example, in U.S. Pat. Nos. 4,779,467 and 5,456,879.
According to the requirements of the “Good Laboratory and Manufacturing Processes” applied to pipettes, monitoring and recording measures should be taken regarding the volume of liquid dispensed. Malfunction of a pipette means that all the tests carried out with the instrument may have to be reconsidered, which constitutes an expensive operation.
Moreover, the quality of a pipette's performance may necessarily fall off over time. After a certain number of operations, each pipette may have to undergo a preventive maintenance procedure. Operators thus have not only to determine after how many pipetting operations such maintenance has to be carried out, but may also keep pipette calibration records.
Finally, account may have to be taken of the fact that the accuracy of the pipette also depends on the operator, who may be more or less skilled and who determines the temperature of the instrument, which influences the volume of aspirated air.